The present invention relates to the field of magnetic resonance artifact correction. It finds particular application in the removal of out-of-slice artifacts in magnetic resonance imaging. It will be described with particular reference thereto.
In spin echo imaging, magnetic resonance is excited with a 90.degree. radio frequency pulse. A 180.degree. RF pulse is applied to cause a subsequent spin echo. The resonance is phase encoded by the application of a phase encoded gradient, either before or after the 180.degree. pulse. Magnetic resonance signals from the echo are collected to form a corresponding view. This sequence is repeated with each of a plurality of phase encode gradient steps to create a plurality of views each with corresponding phase encoding. Commonly, the phase is alternated for adjacent views such that the views have alternating polarity to remove DC offset errors.
Imperfections in the 180.degree. radio frequency pulses create out-of-slice magnetization, particularly in non-averaged spin echo pulse sequences, i.e. sequences in which only one echo signal or view is generated with each phase encoding. This out-of-slice magnetization is not refocused into the echo but its residual signals may occur during data sampling, resulting in artifacted images.
Heretofore, additional gradients have been added to the sequence for dephasing or blurring the out-of-slice magnetization. In addition, phase alternation will shift the out-of-slice artifacts to the edge of the field of view. These correction techniques have several drawbacks. First, the dephasing gradient pulses require a substantial amount of electrical energy. The amount of energy required increases for the higher spatial frequency components which represent edges in the image. Second, the imaging sequence must be lengthened to accommodate the additional dephasing gradients. Third, the minimum field of view must be enlarged. The out-of-slice artifacts that are shifted to the edge of the field of view must be displaced from the edge of the diagnostic image a sufficient distance so as not to interfere. At times this requires generating a larger image than is diagnostically required.
The present invention provides a new and improved magnetic resonance technique which overcomes the above referenced problems and others.